1. Technical Field
This invention relates generally to couplings for connecting pipes together and, in particular, to a transition coupling which is used to join together pipes of different sizes and materials, and which utilizes compression seals between the pipes and the coupling.
2. Background Information
As described in detail in U.S. Pat. No. 4,878,697, it is important that couplings used in natural gas distribution systems using plastic pipe be able to withstand axial stress to avoid losses in sealing capability. Thus, the plastic coupling as described in that patent is concerned with securely gripping the ends of two plastic pipes to hold the pipes together. Grippers within the coupling body grippers are utilized to bite into and grip against the pipes independently of the compression of the seal elements squeezed between the pipes and inside sealing surfaces of the coupling body.
Frequently, in a gas distribution system comprised primarily of plastic gas supply pipe, it becomes necessary to connect the supply pipe to a metal service pipe. In this situation, it is desirable to utilize the same form of coupling employed in connecting together various sections of the supply pipe to also be usable for connecting the plastic supply pipe to the metal service pipe. However, gripping against the outside of a metal pipe which is being connected to a plastic pipe presents a special problem in a substantially plastic coupling.
In the past for metallic couplings, positive gripping of metal pipe is achieved primarily through the use of metallic gripping elements which are wedged to embed into the surface of the metal pipe and hold it against being pulled axially from the coupling and possibly causing a loss in the effectiveness of the seal against the outside of the metal pipe. Such a coupling typically includes a metal body and a metal nut for wedging a metal gripper to bite against the outside of a metal pipe. An example a prior coupling of the foregoing type is shown in U.S. Pat. No. 2,755,109.
Another form of prior art coupling is the transition coupling shown in Rockwell International brochure GCC-800 dated Feb. 1981 at pages 4, 11 and 17. This transition coupling is for use in connecting plastic pipe to metal pipe in a natural gas distribution system. In one important respect, the Rockwell coupling is like the coupling shown in U.S. Pat. No. 2,755,109 and utilizes a metal nut for securing the metal pipe in the coupling.